Optoelectronic packaging

ABSTRACT

Packaging of micromechanical and microelectromechanical devices is carried out by mechanical couplers for connecting pairs or arrays of optical fibers in end-to-end alignment. In another embodiment, a coupler interconnects one or more optical components on a substrate. The electrical components may be active elements such as light sources or light sensors, while the optical components may be waveguides. The fibers are secured in a coupler block, and a substrate carrying the light detector or light source is mounted on or in the block and is secured in alignment with the fibers. The fibers are removably secured within the block by spring fingers.  
     The coupler block may include electrical circuitry connectable to the sensors or light sources on the substrate mounted on the block through wire bonding techniques.

BACKGROUND OF THE INVENTION

[0001] The present invention relates, in general, to micron-scaleoptoelectronic devices, structures and techniques, and more particularlyto devices and structures for facilitating the interaction of opticalcomponents such as optical fibers with other fibers and/or with circuitcomponents such as wave guides or active elements such as light sourcesor light detectors on or connected to micromechanical structures.

[0002] Recent developments in micromechanics have successfully led tothe fabrication of devices in single crystal substrates utilizing a dryetch process such as reactive ion etching (REI) for producingmicron-scale moveable mechanical structures. Such a process is describedin U.S. Pat. No. 5,198,390 as utilizing multiple masks to define small,complex structural elements and related elements such as metal contactsin single-crystal silicon. U.S. Pat. No. 5,393,375 describes a similarprocess for releasing micromechanical structures in single-crystalmaterials other than silicon. An improved dry-etch process for thefabrication of microelectromechanical structures is described in U.S.Pat. No. 5,846,849, which discloses a single-mask, low temperature,self-aligned process wherein discrete devices can be made, and whereinsuch devices can be fabricated in wafers containing integrated circuits.The processes described in these patents may be used to produce avariety of sensor devices such as accelerometers, as well as a varietyof actuator devices, resonators, moveable optical reflectors, and thelike, either as separate, discrete devices or as components onpreviously-fabricated integrated circuits. The processes described inthese patents may be referred to in general as the SCREAM (SingleCrystal Reactive Etch and Metal) process, with the single mask processbeing referred to as the SCREAM-1 process.

[0003] As the field of micromechanical and microelectromechanicaldevices developed, a problem arose concerning the connection of ultrasmall components and structures formed on a wafer or substrate withother circuits and components on other wafers or substrates, whether ofmicron-scale or larger. One solution has been to fabricate solder padson these devices for use in securing connecting lines or wires to theelectrical components on the substrate. However, such a procedurerequires precision wire bonding techniques which do not always producesatisfactory results. Furthermore, the use of wires for communicationwith microcircuits and related devices limits the flow of data betweenthe circuits and devices.

[0004] On the other hand, optical fibers provide many advantages in datacommunication, but problems are encountered in connecting small diameteroptical fibers to micromechanical devices such as waveguides and lightdetectors for transferring data to circuits carried by the substrate, aswell as for transferring data from such circuits, as by way of laserlight sources on the substrate. A major problem is that of alignment offibers with each other, with microstructures such as waveguides andreflectors, with light sources such as vertical cavity surface emittinglaser (VCSEL) arrays, and with electrical circuit components such aslight detectors or the like.

[0005] The alignment of VCSEL arrays and detector arrays for directcoupling to optical fiber arrays is challenging, because the fibers mustbe mounted with their axes perpendicular to the light emitter ordetector. The fiber support structure thus must be perpendicular to thedetector or emitter, and the fabrication of micromechanical supports forthis purpose is difficult.

[0006] Misalignment between fibers or between a fiber and a device orstructure can occur in three translational directions and can occuraround three rotational axes. Optical interconnections are mostsensitive to lateral misalignment; that is, misalignment in directionsperpendicular to the direction of propagation of light in the fiber, butthe connections are also sensitive, to a lesser degree, to angularmisalignment and to the axial distance between components in thedirection of propagation. For single-mode optical systems such as thoseemployed in telecommunications applications, lateral misalignmentbetween optical components should be less than one micrometer, while formultimode systems, lateral misalignment tolerances are more relaxed; forexample, up to about 5 micrometers. In both cases, axial separationtolerances are often greater by a factor 2-5, depending on thecomponents involved. Single-mode interconnections typically can toleratesmall angular misalignments; for example, less than 0.5 degree,depending on coupling efficiency requirements. In the case wherecolumnated beams of light are coupled, where the beam waist is often10-100 times the diameter of typical single-mode fiber beam profiles,angular misalignment of matching beams must be much smaller; forexample, less than 0.01 degree. In all cases an accurate alignment isessential to effective, reliable communication.

[0007] Accordingly, there is a need for structures and devices foraccurately, reliably and easily interconnecting optical fibers with eachother, with micromechanical devices and structures and with lightdetectors and emitters.

SUMMARY OF THE INVENTION

[0008] Briefly, the present invention is directed to improved methodsand apparatus for easily and accurately interconnecting small-diameteroptical fibers in end-to-end axial alignment. The invention is furtherdirected to micron-scale fabrication techniques and to passive opticalcomponents fabricated by such techniques for connecting such opticalfibers to micromechanical and to microelectromechanical devices such aswaveguides and for optically coupling such fibers to electrical circuitsby way of active optical elements such as light detectors and lasersources.

[0009] The packaging of optical fibers with micromechanical andmicroelectromechanical devices is carried out, in a first embodiment, bymechanical couplers for connecting optical fibers in end-to-endalignment so as to obtain a maximum transfer of laser light energy orthe data carried by such light energy from one optical fiber to another.Such couplers may be used to interconnect a single pair of fibers, ormay be used to connect an array of optical fiber pairs, with thecouplers providing easy and accurate assembly.

[0010] In another embodiment of the invention, an optical couplerinterconnects one or more optical fibers with mechanical or electricalcomponents carried by a substrate. The electrical components may beactive elements such as light sources or light sensors, for example,which are electrically connected to corresponding circuit componentssuch as integrated circuits carried by the substrate. Such a coupler mayincorporate trenches for receiving and holding optical fibers inalignment with suitable waveguides or reflectors for directing lightcarried by the optical fibers to corresponding detectors or sensors. Inanother alternative, the circuits or components on the substrate mayconsist of light sources such as a solid state lasers which generatelight in response to signals from electrical circuits on the substrate,with the light produced by the lasers being directed into the opticalfibers by way of the waveguides or reflectors.

[0011] In a preferred form of the invention, alignment of optical fiberswith active optical components such as optoelectric detectors or laserlight sources is attained by securing the optical fiber or fibers in afirst substrate, which will be referred to herein as a coupler block. Asecond substrate, which will be referred to herein as a substrate or awafer, and which contains the light detectors or light sources, issecured to the coupler block. The substrate may be mounted on or above,and parallel to, the surface of the coupler block, with its activeoptical components (light detectors or light sources) positioned inalignment with corresponding fibers. Alternatively, the wafer may beedge-mounted on or in the coupler block, as in a trench formed in thecoupler block, or may be mounted on an edge of the coupler block. Inorder to ensure alignment of the detectors and light sources with theoptical fibers, the trench for receiving the substrate must be preciselyshaped and accurately located, and the substrate must be held firmly inplace. In accordance with the invention, various mounting devices,including fasteners, springs, and the like, are provided to align thesubstrate with the optical fibers in the coupler block and to secure itin place.

[0012] In the preferred form of the invention, the various mountingdevices consist of micromechanical structures fabricated in theconnector block, which preferably is a single crystal silicon substrate.The mounting structures are unitary with the connector block, and arefabricated by one of the SCREAM micromachining processes described aboveso that all of the trenches, connectors, fasteners, springs, waveguides,reflectors, and like structures which make up the connector block of theinvention are fabricated in a single process.

[0013] The SCREAM-1 process utilizes a single crystal substrate of amaterial such as silicon, gallium arsenide, silicon germanium, indiumphosphide, compound and complex structures such as aluminum-gallium,arsenide-gallium-arsenide, and other quantum well or multi-layer superlattice semiconductor materials in which moveable, released structuralelements electrically isolated from surrounding substrate materials andmetallized for selective electrical connections can be fabricated usinga single mask. The structures fabricated by the SCREAM processes can bediscrete; i.e., can be fabricated in a substrate or wafer formed fromany of the aforementioned substrate materials. The processes allowstructures to be fabricated in silicon wafers containing integratedcircuits, since the SCREAM processes use a low temperature dry etchprocedure.

[0014] Complex shapes can be fabricated by the SCREAM processes, asillustrated in the '849 patent, including triangular and rectangularstructures, as well as curved structures such as circles, ellipses andparabolas for use in the fabrication of fixed and variable inductors,transformers, capacitors, switches and the like. Released, cantileveredstructures can be fabricated by this process for motion along x and yaxes in the plane of the substrate, along a z axis perpendicular to theplane of the substrate, and for torsional motion out of the plane of thesubstrate.

[0015] The SCREAM processes in a single crystal substrate permitformation of deep, narrow trenches which may be located and oriented asdesired, and which can be used to define isolated and releasedstructures and to produce high aspect ratio structures. In addition, theprocesses permit deep lateral etching extending below any structureswhich are to be released, and can be used to produce extended cavitiesin the sidewalls of mesas adjacent trenches or surrounding releasedstructures. The released structures can include single or multiplefingers cantilevered to side walls of the substrate and extendingoutwardly over a trench bottom wall, as well as various grids andarrays, and various electrical components. The various structures may bereferred to herein as “beams” or as “released beams”.

[0016] In accordance with the SCREAM-1 process, a dielectric mask layerof oxide or nitride is deposited on the top surface of a wafer orsubstrate, using a standard PECVD process. Preferably, the substrate issingle crystal silicon, with the dielectric layer serving as a maskthroughout the remainder of the steps. The standard PECVD process isused because of its high deposition rate and low deposition temperature.Thereafter, a resist layer is spun onto the mask layer, and standardphotolithographic resist techniques are used to produce in the resistlayer a pattern which defines the desired micromechanical structure. Thepattern in the resist is then transferred to the mask dielectric layerusing, for example, CHF₃ magnetron ion etching (MIE) or RIE. An O₂plasma etch may be used to strip the resist layer, and a deep verticalreactive ion etch (RIE) or a chemically assisted ion beam etch (CIAB) isused to transfer the pattern from the dielectric mask into theunderlying wafer to form trenches which define, in top plan view, theoutline of the desired structures, with the trenches being from 4 to 20micrometers deep and having substantially smooth, vertical walls.

[0017] After completion of the trenches, a protective conformal layer ofPECVD oxide or nitride is applied to cover the silicon structures to athickness of about 0.3 micrometers, for example. The conformaldielectric layer covers the top surfaces of the substrate as well as thesides and bottom walls of the trenches. Thereafter, the conformeddielectric layer is removed from the trench bottom wall, as by ananisotropic RIE which removes the previously applied 0.3 micrometers ofdielectric from the substrate top surfaces and from the trench bottom,leaving the trench side wall coatings undisturbed. As a result, thesubstrate is left with a top surface and side wall layer of dielectric,with the bottoms of the trenches being free of dielectric.

[0018] A deep RIE or CAIBE is used to etch the floor of each trench downbelow the lower edge of the side wall dielectric to thereby expose thesubstrate material below the dielectric on each side of the trench. Anisotropic RIE is then used to etch the substrate material laterallyunder the dielectric layer on the side walls to form cavities. If thetrenches define beams or other narrow structures, the lateral etchingmay extend completely under the beams or narrow structures to releasethem, while cavities will be formed under other fixed (nonreleased)structures, which may be referred to as mesas. The etch chemistry hashigh selectivity to the dielectric, allowing several microns ofsubstrate to be etched without appreciably affecting the protectivedielectric coating. Released beams are thus cantilevered over the bottomwall of the deep silicon trench, with the cantilevered structures havinga core of semiconductor material and a conformal coating of dielectricon their top surfaces and side walls. If desired, a metal layer may bedeposited onto the structure, as described in U.S. Pat. No. 5,846,849.

[0019] The SCREAM-1 process permits fabrication of high aspect ratiomicrostructures with precise geometries, is compatible with existingsemiconductor fabrication techniques, and is preferred, although otherbulk micromachining processes can be used.

[0020] In its simplest form, the optical coupler block of the inventionconnects a pair of optical fibers in an end to end relationship. Thecoupler block is fabricated by a micromachining process such as theSCREAM-1 process to etch a trench, or fiber guide, across a siliconsubstrate or wafer to define the location of the two fibers. The fiberguide is flared where it meets opposite edges of the block to createtapered receptacles which receive the ends of the optical fibers to bealigned and direct them into the fiber guide. The guide dimensions areselected to firmly receive the optical fiber so that when one fiber isinserted into the guide from each end, the fibers will be aligned at thecenter of the guide where they abut. If desired, a precision stop can beetched into the guide to control the distance that each fiber travelswhen being inserted, and a multiplicity of such guides may be formed inthe coupler block to allow alignment of multiple pairs of fibers. Thecoupler block may be fabricated from a stand-alone substrate, or may befabricated by micromachining it in a larger substrate; for example, toform the coupler block in a cavity in the substrate surface.

[0021] In another embodiment of the invention, instead of aligning fiberpairs, the optical coupler block is modified to couple optical fibers topassive optical components such as waveguides, reflectors, or the like,or to active electrooptical components such as light sources or lightdetectors. In this case, a fiber guide is formed so that it extends froma flared receptacle at the edge of a substrate into a correspondingcavity having a vertical wall where the guide terminates. In one form ofthe invention, the cavity is fabricated to incorporate a slopedreflective wall aligned with the optical fiber guide so that lightentering the cavity from an optical fiber in the guide will be reflectedupwardly towards an opening on the surface of the substrate. The couplerblock may support a separate, surface-mounted substrate or wafercarrying an optically active element such as light detector which may bealigned with the upwardly opening reflector in the cavity so that theoptical fiber is in communication with the active element. The separatewafer may contain, or may be connected to, external circuitry or may beconnected to circuitry on the coupler block itself. Alternatively, or inaddition, the optically active element on the separate wafer may be asurface emitting laser which emits light into the cavity when the waferis mounted on the coupler substrate, with the laser light then beingreflected toward the corresponding optical fiber guide.

[0022] In other embodiments, the wafer may be mounted on an end of thecoupler block, with one or more optical fibers extending across thecoupler for alignment with corresponding detectors or surface emittinglasers on the end-mounted wafer. Further, instead of incorporating areflector, the cavity in the coupler may comprise a waveguide forcoupling light from an optical fiber to detectors or to other opticalcomponents on a substrate, or on the coupler block itself.

[0023] In a preferred form of the invention, optical fibers are coupledto active optical elements on a substrate by edge-mounting the substratein the coupler block so that the axes of the fibers are perpendicular tothe substrate surface on which the active elements are mounted. Thisensures that the light from the fibers will strike the surfaces of thecorresponding active elements at right angles, or the light from suchelements will be parallel to the axes of the corresponding fibers, formaximum efficiency. Careful, precise alignment of a substrate carryingoptical elements in or on the coupler block is critical to assuringreliable optical coupling between the optical elements and an opticalfiber, and accordingly a variety of alignment techniques have beendevised, in accordance with the invention. Exemplary techniques andstructures for ensuring accurate alignment of edge-mounted substratesinclude precision etching (i.e., within plus or minus 1 or 2micrometers) of a deep cavity or trench having the dimensions requiredto accurately position an edge-mounted wafer in the coupler block. Thewafer is positioned in the trench with its surface perpendicular to thefibers and with the active elements aligned with corresponding fibers.Once positioned it may be bonded in place, but it has been found thatthermal expansion can cause undue stresses in the microstructures,resulting in deformations which adversely affect optical coupling.Preferably, therefore, the wafer is aligned and secured within thetrench by microsprings fabricated when the trench is formed. The springsmay be provided with tabs or rings to permit retraction for release ofthe wafer, but operate to firmly hold the wafer in a selected positionfor alignment while accommodating changes in dimensions due totemperature variations. The present invention contemplates a widevariety of alignment springs, including edge springs, corner springs andkeyed springs.

[0024] Alignment can be further assured by the provision of notches,pits or depressions formed on the wafer for receiving and locating thealignment springs, and such notches may be tapered or nontapered toreceive corresponding pins or tips fabricated in the connectorsubstrate. If desired, alignment, grooves or trenches can be located onthe wafer to guide the alignment springs into corresponding notches andthe tips of the alignment springs may be tapered, flared or burred tohold them in place. Vertical alignment of the wafer may be provided bysuitable stops or shoulders formed in the etched trench to engagecorresponding notches on the wafer. These techniques can be used toalign one or more wafers in the coupler substrate, as required.

[0025] In order to connect the electrical components carried by thewafer to external circuits, various wire bonding techniques may beutilized, or conventional solder ball interconnections may be used.Thermal stress relief may be provided by mounting the connections onflexible spring-beams, if desired, or the connection can be provided bymeans of a metallized spring tip engaging a contact pad on the wafer.

[0026] The foregoing fabrication and mounting techniques provide acompact electrooptical connector package in which optical fibers areaccurately and reliably aligned with other fibers or electroopticalcomponents carried by a wafer, and to structures wherein opticalcomponents are electrically connectable to corresponding circuitscarried by the coupler substrate or other wafers. Angular and lateralalignment of optical fibers is carried out during the lithographicpatterning steps, in accordance with the invention, so that variouscomponents are effectively self-aligned to a high degree of accuracy.Further, alignment structures such as tips, notches, fiber guides, andthe like, are designed to compensate for variations in etching byproviding symmetrical designs. As a result, when both sides of analignment structure etch at the same rate, the remaining portion isautomatically aligned with a lithographically-determined reference.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The foregoing, and additional objects, features and advantages ofthe present invention will become apparent to those of skill in the artfrom the following detailed description of preferred embodimentsthereof, taken in conjunction with the accompanying drawings, in which;

[0028]FIG. 1 illustrates in a perspective, cross-sectional view anoptical coupler block incorporating etched trenches which define thelocations of optical fiber guides;

[0029]FIG. 2 illustrates the structure of FIG. 1 with the trenchescompleted to form a fiber-to-fiber optical coupler block in accordancewith the invention;

[0030]FIG. 3 illustrates a modified optical coupler block wherein fiberguides include integrated fiber stops;

[0031]FIG. 4 is an exploded perspective view of an optical coupler blockand a surface-mounted substrate which is parallel to a top surface ofthe coupler; the coupler block aligning optical fibers withcorresponding active optical devices on the substrate;

[0032]FIG. 5 is an enlarged, perspective view of a portion of theoptical coupler of FIG. 4;

[0033]FIG. 6 is a diagrammatic, partial cross-sectional view taken alonglines 6-6 of the coupler of FIG. 4;

[0034]FIG. 7 is a diagrammatic, perspective, exploded view of an opticalcoupler block and an end-mounted substrate, the coupler aligning opticalfibers with active optical devices carried on the substrate;

[0035]FIG. 8 is an exploded, perspective partial view of an opticalcoupler block and a surface-mounted substrate, the coupler aligningoptical fibers with edge-emitting optical devices on the substrate,which is mounted parallel to a top surface of the coupler and which islocated in a cavity on the coupler;

[0036]FIG. 9 is a diagrammatic, exploded, perspective, partial view ofan optical coupler block mountable in a carrier for alignment with asurface-mounted substrate carrying optical devices;

[0037]FIG. 10 is a diagrammatic, perspective view in partialcross-section of an optical coupler block for aligning optical fiberswith surface wave guides;

[0038]FIG. 11 illustrates a modification of the device of FIG. 10,incorporating a fiber lens;

[0039] FIGS. 12-17 illustrate successive steps in a process forfabricating a coupler block incorporating an embedded waveguide;

[0040]FIG. 18 is a diagrammatic perspective view in partialcross-section of an optical coupler for aligning optical fibers withembedded and surface waveguides and incorporating multiple fiberalignment fingers;

[0041]FIG. 19 is a side elevation of the embedded waveguide portion ofthe coupler of FIG. 18;

[0042]FIG. 20 is a partial cross-sectional view taken along lines 20-20of FIG. 19;

[0043]FIG. 21 is a side elevation of the surface-mounted waveguideportion of the coupler of FIG. 18;

[0044]FIG. 22 is a partial cross-sectional view taken along lines 22-22of FIG. 21;

[0045]FIG. 23 is a side elevation of a modified form of the device ofFIG. 18, utilizing deposited material on the top of the alignmentfingers;

[0046]FIG. 24 is a partial cross-sectional view taken along lines 24-24of FIG. 23;

[0047]FIG. 25 is a diagrammatic, exploded, perspective view of anoptical coupler and an edge-mounted substrate located in an etchedtrench in the coupler;

[0048]FIG. 26 is a modification of the device of FIG. 25;

[0049]FIG. 27 is a diagrammatic top plan view of the coupler of FIG. 26,illustrating the use of back surface and lateral end alignment springsin the trench;

[0050]FIG. 28 is a diagrammatic, exploded perspective view of a couplerand substrate, illustrating examples of rear and edge alignment springs;

[0051]FIG. 29 is a diagrammatic top plan view of the coupler of FIG. 28with the edge-mounted substrate removed;

[0052]FIG. 30 is a diagrammatic perspective view illustrating theprovision of retracting pins for the springs of FIG. 28;

[0053]FIG. 31 is a diagrammatic top plan view of a modification of thecoupler of FIG. 26, incorporating a back surface alignment spring and apair of left and right edge alignment springs;

[0054] FIGS. 32-35 illustrate in diagrammatic top plan views a varietyof lateral end alignment springs for use with the coupler of FIG. 27,FIG. 32 illustrating a parallel beam edge spring with parallel retractoraction, FIG. 33 illustrating a parallel beam edge-spring withperpendicular retractor action, and FIG. 34 illustrating a serpentineedge spring;

[0055]FIG. 36 is a diagrammatic top plan view of a modification of thecoupler of FIG. 26, incorporating angled corner alignment springs;

[0056] FIGS. 37-39 illustrate in diagrammatic top plan views variouscorner alignment spring structures for use with the coupler of FIG. 36,FIG. 37 illustrating a box beam comer spring, FIG. 38 illustrating aserpentine corner spring, and FIG. 39 illustrating a serpentine cornerspring aligned to notch in an edge-mounted substrate;

[0057]FIG. 40 is a diagrammatic top plan view of a modification of thecoupler of FIG. 26, incorporating front surface alignment springsengaging front surface notches;

[0058]FIG. 41 is a diagrammatic top plan view of an alignment spring foruse with the coupler of FIG. 40;

[0059] FIGS. 42-47 illustrate in diagrammatic top plan views a varietyof alignment tips for use with front surface alignment notches for thecoupler of FIG. 40;

[0060]FIG. 48 is a diagrammatic perspective view of edge alignmentsprings for the coupler of FIG. 31, incorporating edge springs engagingnotches in the front edges of an edge-mounted substrate;

[0061] FIGS. 49-57 illustrate in diagrammatic front elevation views avariety of notch configurations for the edge-mounted substrate of FIG.48;

[0062] FIGS. 58-67 illustrate variations in the etched alignment notcheswhich are centered on an end wall of a substrate or on a front surfaceof the substrate for use with the embodiments of FIGS. 27 and 40,respectively;

[0063]FIG. 68 is a partial perspective view of a modified etchedalignment trench centered on an end or on a front surface of anedge-mounted substrate incorporating a notch for receiving a taperedalignment tip;

[0064] FIGS. 69-72 illustrate modified forms of the alignment trench ofFIG. 68;

[0065]FIG. 73 illustrates in perspective view another modification ofthe alignment trench of FIG. 68, utilizing a square-ended alignment tipand using etched V-grooves;

[0066] FIGS. 74-75 illustrate additional modifications of the alignmenttrench of FIG. 68 for receiving outwardly flared alignment tips;

[0067]FIGS. 76 and 77 illustrate in elevation view the shape of thealignment trench of FIG. 74 and a modification thereof respectively;

[0068]FIG. 78 is a diagrammatic perspective view of the flared alignmenttip and corresponding alignment trench of FIG. 74;

[0069] FIGS. 79-81 illustrate the use of alignment tips with burrs forthe front surface alignment notches of FIG. 40 and modificationsthereof;

[0070] FIGS. 82-84 illustrate variations of the burred alignment tipsutilized in FIGS. 79-81;

[0071]FIG. 85 illustrates in perspective view the alignment tip andcorresponding notch for the edge-mounted substrate of FIG. 80;

[0072]FIG. 86 illustrates in diagrammatic top plan view a substratehaving a through hole for receiving front and rear surface alignmenttips;

[0073] FIGS. 87-91 illustrate through holes of various shapes anddimensions combined with alignment trenches of the type illustrated withthe substrate of FIG. 58;

[0074]FIG. 92 is a diagrammatic perspective view of the device of FIG.86, having a through-hole engaged by front and rear surface taperedalignment tips;

[0075]FIG. 93 is a diagrammatic, perspective, exploded view of amodification of the coupler of FIG. 28, illustrating vertical alignmenttrenches for receiving alignment stops and front and rear surfacealignment trenches for receiving corresponding alignment springs, andincorporating the optical fiber alignment fingers of FIG. 18;

[0076]FIG. 94 is a side elevation of a portion of the device of FIG. 93;

[0077]FIG. 95 is a cross-sectional view taken at line 95-95 of FIG. 94;

[0078]FIG. 96 is a diagrammatic partial perspective view of amodification of the device of FIG. 93;

[0079]FIG. 97 is a side elevation of the device of FIG. 96, illustratingthe modified fiber alignment finger springs,

[0080]FIG. 98 is a cross-sectional view taken along lines 98-98 of FIG.97;

[0081]FIG. 99 is a top plan view of an optical coupler having severalcomponents aligned with each other;

[0082]FIG. 100 is a modified form of the optical coupler of FIG. 99; and

[0083] FIGS. 101-106 illustrate various structures for providingelectrical interconnections between an optical coupler and anedge-mounted substrate.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0084] Turning now to a more detailed consideration of the presentinvention, FIGS. 1 and 2 illustrate steps in the fabrication of anoptical coupler generally indicated at 10, for providing end-to-endalignment of pairs of optical fibers. In the preferred form of theinvention, the coupler is fabricated from a substrate, or block, 12 of asuitable material such as single crystal silicon. One or more trenches,such as the trenches 14 and 16, are first formed on the top surface 18of block 12, using, for example, the SCREAM process described above.Thus, the trenches are anisotropically etched across the top surface ofthe substrate block 12 to define the locations of fiber guides which areto be formed. The trenches preferably extend completely across the topof the substrate 12, from side wall 20 to side wall 22, with thetrenches being flared as indicated at 24 and 26 where they meet thesidewalls of the substrate. The flared trenches permit formation offlared fiber guide ends in the next step of the process.

[0085] It will be understood that a single trench may be provided topermit end-to-end alignment of a single pair of optical fibers, ormultiple trenches can be provided to permit alignment of an array ofoptical fiber pairs. For simplicity, the trenches preferably areparallel to each other across the coupler block 12, but other alignmentsare possible, if desired, since the use of a single crystal material forthe substrate 12 permits formation of the trenches without regard to thecrystal structure.

[0086] Although not illustrated in the Figure, the side walls of thetrenches 14, 16 are covered with a protective layer of oxide so that, asindicated in FIG. 2, fiber guides 30 and 32 can be fabricated byisotropically etching away the bottoms 34 and 36, respectively, oftrenches 14 and 16. The dimensions of the resulting guides 30 and 32 aredetermined by the width and depth of the anisotropically etched trenches14 and 16, by the reactive ion etch time, by the plasma potential usedin the etch process, by gas flow and chemistry, as well as otherparameters known from the SCREAM-1 process. Since typical optical fibershave an outer diameter of about 125 micrometers, the etch parameters andresulting trench structure dimensions are selected to create fiberguides 30 and 32 having corresponding compatible dimensions. Thus, forexample, the center portion 38 of each fiber guide would have a diameterequal to or slightly larger than the diameter of the optical fiberswhich it is to receive, while the guides would have flared end portions40 and 42 large enough to facilitate insertion of the fibers into thecoupler.

[0087] If desired, a fiber stop structure 44 can be fabricated in eachof the fiber guides 30 and 32, as illustrated in FIG. 3. These arefabricated by shaping the trenches 14 and 16 to incorporate narrowedregions 46 and 48 to restrict the etching and to thereby produce thering 44 of unetched material. This ring 44 reduces the diameter of thefiber guide and acts as a precision stop for inserted fibers.

[0088] In operation, two fibers of a pair of optical fibers which are tobe aligned in end-to-end relationship are inserted into a fiber guide,such as the guide 32, from opposite ends and are pushed into the guideuntil they reach opposite sides of the fiber stop structure 44. The twooptical fibers are thereby coaxially aligned with the guide 32 and witheach other, and thus are aligned for transfer of light from one fiber tothe other with minimal loss. In this simplest form of the invention, theoptical coupler aligns pairs of fibers coaxially in end-to endrelationship. It will be apparent that the fibers may be secured in theguides by providing a close tolerance between the diameters of thefibers and the guide, or they may be secured in place by an adhesive orother fastener, as required.

[0089] The coupler of the present invention may be modified to secureand align optical fibers not only with other fibers, but also withexternal structures or components which may be mounted on or secured tothe coupler. Thus, for example, FIG. 4 illustrates a modified opticalcoupler 50 having on its upper surface 51 a plurality of trenches 52-55through which corresponding fiber guides 58-61, respectively, arefabricated utilizing, for example, the SCREAM-1 process. These fiberguides receive the optical fiber array 62 which consists of fibers64-67, respectively, with the diameter of the fiber guides beingselected to firmly receive and hold the respective optical fibers. Ifdesired, fiber stops, such as the stop structure 44 illustrated in FIG.3, may be provided in each of the guides to provide a positive locationfor the inner end of each of the fibers.

[0090] In the embodiment of FIG. 4, the fibers are to be aligned with acorresponding array of optically active devices, such as opticaldetectors or laser light sources, indicated at 70-73, mounted on asubstrate, or “flip-chip” 76. The flip-chip 76 is mounted on the topsurface 51 of an optical coupler block 78 by any suitable means so thatthe substrate 76 is parallel to the top surface 51. In the illustratedembodiment, the surface mounted substrate 76 carries a plurality ofmetallized mounting pads 80 which are aligned with and engagecorresponding solder balls 82 carried on metallized pads 84 on the topsurface 51 of block 78. The pads 80 are aligned along a first edge 86 ofthe substrate; similarly, metallized pads 80′ are aligned along anopposite edge 88 of the substrate 76 in alignment with correspondingsolder balls 82′ mounted on pads 84′ on the surface 51. The pads 84′ maybe connected through suitable surface conductors 90 to circuitry 92 onthe surface 51 of the block 78, for connecting the circuitry to theactive elements 70-73 to drive light sources or to receive signals fromdetectors. The driver or receiver circuitry 92 may be in the form of anintegrated circuit fabricated on the coupler block 78 using conventionalintegrated circuit technology. The active elements 70-73 may beconventional vertical cavity surface emitting lasers (VCSELs) or may beconventional light detectors, depending on the use to which theflip-chip is to be put.

[0091] To secure the substrate 76 to the coupler, the entire assembly isplaced in a reflow oven to melt the solder balls. This causes the solderto pull the substrate 76 into alignment with the metal pads on thesurface of the optical connector 50. For solder balls with a diameter of50 micrometers on metallized pads 50 micrometers in diameter, lateralmisalignments of less than about 0.5 micrometer can be obtained;accordingly, the solder pads allow submicron tolerances in the alignmentof the active devices 70-73 with the optical fiber array. In addition tothe mechanical alignment, the solder balls provide electricalconnections and a thermal dissipation path for the active devices.

[0092] In accordance with the embodiment of FIG. 4, light is transmittedbetween corresponding optical fibers 64-67 and active devices 70-73 byway of precision reflectors etched in cavities 100-103 formed in thecoupler block 78 at the end of corresponding fiber guides. The cavities100-103 are identical, and are exemplified by the cavity 101 illustratedin cross section in the enlarged views of FIGS. 5 and 6. As thereillustrated, the cavity 101 is located at the end of fiber guide 59 andis fabricated as an inverted pyramid having four sloping sidewalls106-109 which taper inwardly and downwardly to an apex 110. The fiberguide 59 extends to the sidewall 108 and provides an aperture 111 inthat wall so that when the optical fiber 65 is positioned in guide 59(FIG. 6), the inner end 112 of the fiber will extend through aperture111 and slightly into the cavity, with the fiber aligned with theopposite wall 106. The cavity walls are reflective, as by metalization,so that light entering the cavity from the optical fiber 65 will bereflected from wall 106 onto a corresponding light detector 71 carriedby substrate 76. Similarly, if the active element 71 is a light emitter,then light from that device will be directed onto reflective wallsurface 106 and from there into the optical fiber 65. Accordingly, thecoupler 50 precisely aligns the optical fibers 64-67 with correspondingactive elements 70-73 by way of corresponding reflector cavities 100-103to couple light between the substrate 76 and the optical fiber array,with the reflective surface causing the light paths to be perpendicularto the active elements.

[0093] A further modification of the optical connector of the presentinvention is illustrated at 118 in FIG. 7, wherein a substrate 120carrying an array of active elements such as the elements 122-125 is tobe mounted on the surface of an optical coupler block 130 without theneed for etching cavities with angled reflector faces of the typedescribed above. This is accomplishing by extending fiber guides 132-135completely across block 130, so that the guides extend from sidewall 138to opposite side wall 140 of the block. The corresponding optical fibers142-145 are positioned in the respective guides with their inner endfaces substantially flush with end wall 140. The substrate 120 issecured to wall 140 by solder balls 150 mounted on correspondingmetallized pads 152 on the wall 140. Corresponding metallized pads 152′are precisely located on the substrate 120 so that when the substrate120 is positioned on the solder balls and the solder is reflowed, thesubstrate will be precisely positioned on the end wall 140. As discussedabove with respect to FIG. 4, this mounting will precisely align theoptical fibers with the corresponding active elements on substrate 120,with the fibers perpendicular to the substrate surface.

[0094] As illustrated, the active elements 122-125 may be electricallyconnected through conductors to corresponding contact pads 154-157 onthe substrate 120. These pads may be electrically connected tocorresponding wire bonding pads 158 through 161 located on the topsurface 164 of block 130, with the metal contacts being connected to thepads by conventional right-angle wire bonding. The wire bond pads maythen be connected to external circuitry or to integrated circuits on thesubstrate 130, as desired.

[0095] Since the fiber guides 132-135 are buried beneath the surface 164of block 130, in another embodiment the substrate 120 can be surfacemounted in a cavity formed on the surface of the coupler block 130, asillustrated in FIG. 8, to avoid the difficulty of mounting the substrateon the end wall 140 as described above. In this embodiment, thesurface-mounted substrate 160 is illustrated as carrying Fabry-Perotlasers 162-165 arranged along one edge 168 of the substrate. A cavity170 is fabricated in the top surface of block 130, for example, by theSCREAM-1 process described above, with the depth of the cavity beingdetermined by the depth of the longitudinal axes of the optical fibers142-145 and by the location of the lasers 162-165 on the substrate 160.A plurality of metallized pads 172 and 172′ are placed on the lowersurface 173 of substrate 160 in alignment with corresponding metallizedpads 174 and metallized connectors 176 on the floor 178 of cavity 170.Solder balls 180 are located on each of the metallized pads andconnectors which, when they are reflowed, provide a mechanical andelectrical connection between pads 172 and pads 174, or pads 172′ andpads 176, to precisely align substrate 160 in the cavity 170 so that thelasers 162-165 are precisely aligned with the end faces 182-185 of thefibers 142-145. The metallized pads 172 are mechanically connected tocorresponding pads 174 through solder balls 180 primarily for thepurpose of securing and aligning the substrate 160, while metallizedpads 172′ are aligned with corresponding connector pads 176 primarily toprovide electrical connections through the solder balls 180 to connectthe lasers 162-165 to laser driver circuitry 190 which may be carried onthe surface of substrate block 130.

[0096] As noted above, the depth of the cavity 170 is determined by thedepth of the axes of the optical fibers when they are located in theircorresponding fiber guides and by the rest height of the laser array onsubstrate 160 after reflow of the solder balls 180. Laser array heightvariations can be reduced by using additional solder balls forsupporting the substrate 160, but strict control on solder ball volumeas well as control and repeatability of the cavity etch depth isrequired to obtain precise optical alignment. Careful location of themetallized pads reduces the chance of lateral misalignment in the deviceof FIG. 8.

[0097] Another embodiment of a laser-fiber optical coupler isillustrated in FIG. 9, wherein a coupler block 200 is in the form of a“flip-chip” which includes on its top surface 202 a multiplicity ofmetallized pads 204 which are used for mounting the block 200 by meansof corresponding solder balls, as described above. Also formed in thesurface 202 of block 200 are a multiplicity of spaced, parallel fiberguides 206-209 fabricated as described above to receive correspondingoptical fibers 216-219. In this embodiment, the fibers extend completelythrough the block 200 with the end faces of the fibers being flush withthe end wall 220 of block 200, as illustrated at 226-229.

[0098] The block 200 is received in a cavity 230 formed in the topsurface 232 of a suitable wafer or the like generally indicated at 234.The cavity 230 includes a multiplicity of metallized pads 236 which arealigned with the pads 204 when the coupler block 200 is positioned inthe cavity. Solder balls 240 are positioned on each of the pads 236 andwhen they are reflowed, they serve to secure and accurately positioncoupler 200 in cavity 230.

[0099] In this embodiment, the substrate 160 may be mounted on the topsurface 232 of wafer 234, with metallized pads 172 and 172′ on thebottom surface of substrate 160 being aligned with corresponding pads174 and 176 located on the top surface 232. Solder balls 180 on pads 174and 176 serve to secure and accurately position the substrate 160 sothat the lasers 162 through 165 will be aligned with the end faces ofthe corresponding fibers 216-219. The metallized connector pads 176 onsurface 232 connect the lasers 162-165 through pads 172′, solder balls180, and the connector pads to laser driver circuitry 190, also mountedon surface 232. The flip-chip mounting of both the fiber guides in block200 and the laser array on substrate 160 in the manner illustrated inFIG. 9 reduces the incidence of misalignment between the lasers and theoptical fibers in a direction perpendicular to the surface of wafer 234caused by variations in height due to variations in solder ball size.Although lateral misalignment parallel to the surface 232 is possible,such misalignment is minimized by biasing both the coupler 200 and thesubstrate 160 in the same direction prior to reflow so that the reflowprocess tends to keep them in alignment. However, the depth of thecavity 230 must be accurately controlled to maintain alignment in thevertical direction.

[0100] FIGS. 10-24 are directed to embodiments of the optical coupler ofthe present invention for aligning optical fibers with opticalwaveguides. In particular, these embodiments relate to fiber-waveguideoptical couplers fabricated in a coupler block such as a siliconsubstrate for receiving light from optical fibers and coupling thatlight into a receiving waveguide which may either be on the surface ofthe optical coupler or embedded in the coupler. One embodiment isillustrated in FIG. 10, wherein an optical coupler 248 includes acoupler block 250 which incorporates an array of fiber guides, such asthose illustrated at 252 and 254, which are etched into the block in themanner described with respect to FIGS. 1-3. As previously described, thefiber guides are formed in the top surface 256 of the block 250, withthe guides incorporating flared ends 258 and 260, as previouslydescribed.

[0101] Mounted on the surface 256 of coupler block 250 is an array ofreceiving waveguides such as the waveguides 262 and 264. Thesewaveguides are fabricated by depositing on surface 256 suitablewaveguide materials such as glass, polymer, or lithium niobate, forexample. This material is patterned using standard semiconductorfabrication processes, with the resulting waveguide leading to photonicdevices (not shown) to which the optical signals carried by opticalfibers in fiber guides 252 and 254 are to be coupled. The receivingwaveguides 262 and 264 are coupled to and aligned with the opticalfibers by means of deep-etched tapered cavities, or trenches, formingwaveguide couplers 266 and 268, respectively, in the substrate block250, as will be described.

[0102] The tapered waveguide couplers 266 and 268 are fabricated byestablishing deep etch conditions which make wider trenches also deeperso that their vertical and horizontal dimensions change gradually overtheir length. Each cavity is filled at least partially with an opticalwaveguide material 269, such as glass or polymer, and achemical-mechanical polish (CMP) is used to make the top surface smooth.Next, the waveguide material for surface waveguides 262 and 264 isdeposited and patterned, and after this the fiber guides 252 and 254 areetched. If desired, the same waveguide material 269 can be used for boththe trench filler and the surface waveguides.

[0103] The large ends 270 and 272 of the waveguide couplers 266 and 268are adjacent the fiber guides 252 and 254, respectively. For clarity ofillustration, these ends 270 and 272 are shown as being wider and deeperthan is necessary. In practice, the taper of waveguide trenches 266 and268 may be designed to support a single mode in a plane parallel to thetop surface 256 of block 250, with the profile of the guided mode in thewaveguide closely matching the profile in the optical fiber to reducecoupling losses. While it is easy to profile the width of the taperedend to achieve a profile match, setting the thickness of the waveguidematerial is more difficult due to the aspect ratio of the taperedregion. Such a taper will support many modes in the plane perpendicularto the top surface 256, so the tapered trenches are more suited to forcoupling to multimode fibers, where an appropriate match between thetapered region and fiber mode field profiles are easier to fabricateusing the here-in described processing procedures. A shorter taperminimizes pulse spreading but the geometry of the taper is of lessconcern for multimode coupling. Due to the tall shape of the waveguidecouplers 266 and 268, these couplers are primarily applicable forcoupling from optical fibers into planar waveguides such as thoseillustrated at 262 and 264.

[0104] Coupling from planar waveguides, such as the waveguides 262 and264, into corresponding optical fibers is illustrated in FIG. 11,wherein elements similar to those of FIG. 10 are similarly numbered. Inthis embodiment, the surface waveguides 262 and 264 are coupled tocorresponding deep-etched waveguide couplers 280 and 282 which arenarrower than the waveguide couplers 266 and 268, and which are taperedoutwardly from an inlet end 284 to an outlet end 286. In thisembodiment, light from the waveguides 262 and 264 passes throughcouplers 280 and 282 and is focused into the corresponding opticalfibers in fiber guides 252 and 254, using an optical fiber 290 as acylindrical lens. As illustrated, the fiber 290 is located in adeep-etched trench 292 which extends transversely across the substrateblock 250, intersecting the waveguide couplers 280 and 282 where theyjoin the ends of the fiber guides 252 and 254. The relative positionsand depths of the tapered waveguide couplers, fiber lens, and receivingoptical fibers in guides 252 and 254 are selected to maximize opticalcoupling. An integrated fiber stop, such as the stop 44 illustrated inFIG. 3, can be provided in the fiber guides 252 and 254 to ensure properlens-fiber spacing.

[0105] In a modification of the waveguide couplers of FIGS. 10 and TI, asingle-mode fiber-to-waveguide coupler 298 may be fabricated byproviding a thin, linear (i.e., non-tapered) waveguide in a cavity fordirecting light between a surface waveguide and an optical fiber whichintersects the cavity. Such a thin waveguide may be fabricated inaccordance with the process steps illustrated in FIGS. 12-17. In thefirst step, (FIG. 12), the upper surface 300 of a coupler block 302,which may be single crystal silicon, is patterned and etched with anisotropic deep etch to create a waveguide coupler cavity 304. Thiscavity is relatively large at a first end 306, and tapers inwardly andupwardly to a reduced end 308 which leads to and is integral with asurface channel 310. The desired coupler waveguide material 312 is thenconformally deposited on the top surface 300 and in the waveguide cavity304, and the material 312 on the top surface is chemical-mechanicalpolished to reveal the substrate top surface 300, leaving a layer of thematerial 312 coating the interior walls and floor of the coupler cavity304 and filling the channel 310.

[0106] Thereafter, as illustrated in FIG. 14, waveguide material for asurface waveguide 314, which leads to photonic devices or circuits onsubstrate 302, for example, is deposited and patterned to produce thesurface waveguide. This step can be merged with the previous step ofFIG. 13 if the waveguide material characteristics and depositiontechniques for the structure 314 and the material 312 are compatible. Ifdesired, a coupler structure, taper, or grating can be fabricated wherethe two waveguides overlap at the channel 310 to improve coupling.

[0107] The next step, illustrated in FIG. 15, is to fabricate a fiberguide 320 by isotropic etching, as described with respect to FIGS. 1-3.Thereafter, as illustrated in FIG. 16, the material 312 in waveguidecoupler cavity 304 is patterned, as illustrated by dotted line 322, todefine a linear waveguide and, as illustrated by dotted line 324, todefine a trench region. Following the patterning, an isotropic deep etchis performed to produce a deep trench 326, as illustrated in FIG. 17,surrounding a linear waveguide 330 as patterned at 322 in material 312.The deep etch forms an upstanding ridge 332 beneath, and supporting,waveguide 330. The depth of the fiber guide 320 and the depth of thecoupler cavity 304 are selected so that the axis of an optical fiberinserted into the fiber guide 320 will be aligned with the center oflinear waveguide 330. In this way, light from the optical fiber isguided through waveguide 330 into the surface waveguide 314 or viceversa. If desired, the etched cavity can be filled with a claddingmaterial to improve the coupling between the optical fiber and thewaveguide.

[0108] In the embodiments described above, the fibers are secured infiber guides located below the surfaces of the respective opticalcoupler blocks and are held in place by friction between the fiberguides and the surfaces of the optical fibers. In another embodiment ofthe invention, illustrated at 339 in FIG. 18, shallow fiber guides 340are fabricated in a coupler block 342. In the illustrated embodiment,the depth of the fiber guides is selected to align the center of anoptical fiber, such as the fiber 344, with a buried waveguide 346located in a trench 348 etched in the surface 350 of block 342. Thus,the depth of the fiber guide 340 is selected to be a little larger thanthe radius of the optical fiber 344 so that the surface of the fiberextends above the top surface 350 of block 342, as illustrated.

[0109] In this embodiment, the fiber guide 340 is fabricated so that itswidth is greater then the diameter of the fiber 344, and a plurality offiber alignment finger springs 352 and 354 are provided which extendfrom opposite sides of the trench from which the fiber guide 340 isformed. The finger springs are undercut, released, cantilevered beamspreferably fabricated by the SCREAM-1 process and extend toward eachother and toward the center of the fiber guide 340 to receive and engagean optical fiber and to bold it in alignment with the waveguide 346. Asillustrated in FIG. 18, the depth of each of the finger springs 352 and354 is approximately equal to its width so that the fingers are flexibleboth vertically and horizontally to permit easy insertion of the opticalfiber into its corresponding fiber guide. The fingers have sufficientthickness to ensure that the fiber is restrained. Thus, the amount ofpressure required to insert and align the optical fiber depends on thefinger spring geometry and the number of fingers. These finger springsare located lithographically in the fabrication process, and are alignedin opposition across the fiber guide.

[0110] A similar embodiment is illustrated in FIGS. 19 and 20; however,instead of using an undercutting etch step to fabricate thin fingers asin FIG. 18, relatively thick alignment fingers 356 and 358 arefabricated in a fiber guide 360, located in a coupler block 362. In thiscase, the fingers 356 and 358 are relatively narrow so that they areflexible in the horizontal direction, and are relatively thick in thevertical direction. These thick fingers provide greater stiffness andthus provide improved alignment over the thin fingers of FIG. 18. Theoptical fiber 364 is secured in the fiber guide 360 by the fingers 356and 358, with the surface of the optical fiber extending above the topsurface 366 of block 362 so that the axis of the fiber is aligned with aburied waveguide 346.

[0111] If a waveguide 370 is on the surface of the coupler block, asillustrated in FIGS. 21 and 22, a fiber guide 372 in the surface 374 ofblock 376 will have to be shallower in order to align the axis ofoptical fiber 378 with the waveguide. In this case, the alignmentfingers 380 and 382 on opposite sides of the waveguide will also beshallow, with the tops of the fingers being below the axis of theoptical fiber, making it difficult to retain the optical fiber in thefiber guide 372 since the fingers would tend to squeeze the fiber out ofthe guide. This would lead to lateral misalignment of the optical fiberwith the waveguide. One solution for this is illustrated in FIGS. 23 and24, wherein an additional layer 384, is deposited or epitaxially grownon the top surfaces of alignment fingers 380 and 382, respectively. Thislayer grips the fiber securely to reduce the possibility ofmisalignment.

[0112] As discussed above, the alignment active optical devices such asVertical Cavity Surface Emitting Laser (VCSEL) arrays and photo detectorarrays which are to be optically coupled to optical fiber arrays ischallenging, because the light to be detected must be perpendicular tothe light-detecting elements or the emitted light must be parallel tothe optical axis of the fiber. In the previously-described embodimentsin which the active elements are mounted on a separate, surface-mountedsubstrate, as in FIGS. 4-6 and 8-9, a coupler is provided in which lightis redirected, as by reflective surfaces or waveguides, between theactive elements and optical fibers which are not perpendicular to theelements. In accordance with the present invention, another solution tothis alignment problem is to edge-mount the substrate which carries theactive elements in a cavity so that the surface of the substrate whichcarries the active elements is perpendicular to the optical fibers. Suchan arrangement is illustrated at 398 in FIG. 25, wherein a substrate 400is mounted on one edge in an etched trench 402 located in a siliconfiber guide substrate, or coupler block, 404. The coupler block 404includes an array of fiber guides, generally indicated at 406, whichreceive an array of optical fibers 408, as described above with respectto FIG. 4, for example. The substrate 400 carries an array of activeoptical elements such as VCSELS or detectors, which emit or detectlight. In the illustrated embodiment, the elements 410 are VCSELS whichemit light through the substrate 400. These VCSELS are to be alignedwith corresponding optical fibers in array 408, and for this purpose thesubstrate 400 is positioned in the deep etched trench 402 and is held inplace by shaping the ends of the trench, as at ends 412 and 414, tosnugly receive corresponding ends of the substrate.

[0113] The substrate 400 carries a plurality of wire bond pads 416 whichmay be electrically connected to suitable driver circuitry 420 carriedon the top surface 422 of coupler block 404 by way of correspondingconnector pads 424, also on surface 422. Conventional wire bonding, asdescribed with respect to FIG. 7, for example, may be used to providethe electrical connections. By controlling the depth and position of thetrench 402 and the location of the active elements 410 on substrate 400,accurate alignment of the active elements with the optical fiber arraycan be obtained. The substrate so mounted in the coupler block may bereferred to as an edge-mounted substrate.

[0114]FIG. 26 illustrates a variation of the device of FIG. 25, whereinthe optical coupler block 404 receives the optical fiber array 408 incorresponding optical fiber guides 406, in the manner previouslydescribed. In this case, the active elements on the substrate 400 areillustrated as optical detectors 424 located on the surface 425 of thesubstrate which faces the optical fiber array 408. The substrate 400 isreceived in trench 402 in the manner previously described. In this case,the wire bond connector pads 416 are located on the surface 425 ofsubstrate 400 which faces the optical fiber array and are connected tocorresponding wire bond connector pads 428 on the surface 422 ofconnector block 404. Electrical connections to detector circuitry aremade through additional wire bond connections to pads 428.

[0115] In the devices of FIGS. 25 and 26, the active devices on theedge-mounted substrate are aligned with the optical connector block 404and thus with the optical fiber array 408 by precision etching of thedeep etch trench 402 and by precision cleaving of the edge-mountedsubstrate so that the substrate has the correct dimensions. Precisioncleaving of the substrate 400 may be attained by etching small trenchesaround its parameter to establish the precise location of the cleavededges. The deep trench etch depth must be controlled to within 0.5micrometers for successful alignment to single-mode optical fibers.After the edge-mounted substrate is inserted into the trench, it isbrought into contact with one end of the trench and is fixed in positionwith an adhesive such as epoxy or by some other bonding method.

[0116] One problem with the foregoing assembly technique is that it maynot adequately address expansion effects arising from differentcoefficients of thermal expansion of the edge-mounted substrate and theoptical coupler 404. FIG. 27 diagrammatically illustrates a solution tothis problem wherein an edge-mounted substrate 430 (which may be thesame as substrate 400, for example) is secured in a deep-etched trench432 in an optical coupler block 434 by means of microsprings which bendslightly to accommodate thermal expansion and contraction of theedge-mounted substrate relative to the coupler block. These microspringsare located, as indicated by arrows 436 and 438, to press the substratetoward the optical fibers and toward the left-hand end of the trench432, respectively, as viewed in FIG. 27. The left-hand end of the trenchmust still be etched to sub-micron tolerances to ensure alignment of theactive elements 440 with the optical fibers in fiber guide 442, but thesprings accommodate relative motion due to differences in thermalexpansion.

[0117]FIG. 28 illustrates in perspective view examples of back surfaceand lateral alignment springs 436 and 438, while FIG. 29 illustrates atop plan view of the coupler block 434 with the substrate 430 removedfor clarity. In addition, the back surface spring 436′ in FIG. 29 is ofslightly different configuration to illustrate that the springs can takea variety of forms.

[0118] The various alignment springs are fabricated in the siliconcoupler block using the SCREAM-1 process by first patterning the topsurface 444 of block 434 at the same time that trench 432 is patterned.After the trench 432 and the springs have been etched to the desireddepth, the springs 436 and 438 are released for motion with respect tothe block 434 by deep etching the block 434 from its back surface 446.This back surface etch takes place in the regions indicated by thedotted lines 448 and 450 in the top plan view of FIG. 29, leaving spring436 connected to substrate 434 by cantilever arms 452 and 454 andleaving spring 438 connected to substrate 434 by cantilever arm 456. Anadvantage of releasing by deep etching from the back side of thesubstrate is that the back surface etch depth does not require strictcontrol, since any over-etch in this location only results in a springwhich is slightly thinner than desired. Since the SCREAM-1 process iscapable of fabricating high aspect ratio structures such as springs 436and 438 which are tens of microns deep, any over-etch on the backsurface on the order of micrometers will not significantly influence theoperation of the resulting springs.

[0119] As illustrated, spring 436 includes a base region 460 connectedto cantilevered arms 452 and 454 through sinuous connector arms 462 and464 which allow motion toward and away from the optical fiber guides442. This spring tends to urge the substrate 430 into position againstthe fiber guides. Similarly, released edge alignment spring 438 includesan edge contact arm 466 mounted on cantilever arm 456, with the arm 456being movable toward and away from the end of substrate 430 to urge thesubstrate toward the left-band end of the deep trench 432, as viewed inFIG. 29.

[0120] The alignment spring 436 is illustrated in FIG. 30 as having aring 470 (FIG. 30) which may be used to retract the spring duringinsertion of the edge mounted substrate 430 into the trench. Similarly,alignment spring 438 incorporates a tab, or extension 472 for the samepurpose. A substrate insertion tool 473 may incorporate correspondingtapered retraction pins 474 and 476 which automatically retract thesprings 436 and 438, respectively, during the substrate installationprocess, and release the springs after the substrate is in place. Theinsertion tool grips the substrate for precision placement in trench432. Alternatively, the pins can be independent of the substrateinsertion tool. As illustrated in FIG. 30, the pins 474 and 476 includesloped faces 478 and 480 which engage ring 470 and tab 472 to force thesprings away from the substrate as the pins are lowered.

[0121] A variety of alignment spring structures may be utilized toassist in aligning the edge-mounted substrate 430 in trench 432 of block434. Such alignment structures are diagrammatically illustrated in FIG.31 as including not only the back surface alignment spring 436previously discussed, but also variations in the lateral alignmentspring 438 on the right-hand side of the substrate 430 as well asalternative edge alignment springs 490 illustrated in FIG. 31. Suchvariations and modifications are illustrated in FIGS. 32-39, to whichreference is now made.

[0122]FIG. 32 illustrates a lateral alignment spring 491 which issimilar to spring 438 of FIG. 30. This spring includes an alignment tip492 which engages an end of the substrate 430, and is mounted on a pairof parallel spring arms 494 and 496 which are mounted as cantilevers onthe wall portion 498 of an extension 499 of trench 432. As illustratedby dotted lines 450, the alignment spring 491 is released by a deeptrench etch on the back surface of the substrate 434, as was describedwith respect to FIG. 29. The spring arms 494 and 496 tend to urge thetip 492 against the end of substrate 430 to hold the substrate in placewhile accommodating changes due to temperature. The spring may bereleased from the substrate, or moved to the right as viewed in FIG. 32,to permit insertion of the substrate, by means of a retractor pin 500which may be utilized in the manner described above with respect to pins474 and 476 in the device of FIG. 30.

[0123] As illustrated, in the rest position of alignment spring 491 thetip 492 extends past the location of the substrate 430, (indicated bydotted lines in FIG. 32). It will be understood that when the substrate430 is in place, the tip of the spring 491 will engage the edge of thesubstrate, shifting the spring toward the right, as viewed in FIG. 32.The spring arms 494 and 496 will then urge the spring toward the left,against the edge of the substrate to hold it in place.

[0124]FIG. 33 illustrates at 501 a slightly modified form of thealignment spring of FIG. 32. In this case, the alignment spring 501includes a shoulder portion 502 which receives the force supplied byretractor pin 500 to move the spring toward the right to permitinsertion of substrate 430 or to release the substrate. In thisembodiment, pin 500 engages a fixed beam 504 which extends outwardlyfrom wall portion 498 of trench extension 499 in cantilever fashion toprovide a support for the retractor pin. The pin may then be movedhorizontally into a tapered slot 506 formed between shoulder 502 andbeam 504 to shift the alignment spring.

[0125] The lateral alignment spring may be fabricated to have theserpentine configuration illustrated for back spring 436 in FIG. 30.Thus, as illustrated in FIG. 34 a lateral alignment spring 508 mayinclude an alignment tip 510 secured on a base 512 mounted to the wallof trench 432 by serpentine springs 514 and 516. The base 512 may alsosupport a retractor ring 518 which receives the retractor pin 500 forshifting the edge alignment tip 510 to the right to permit insertion ofa substrate 430 or to release it.

[0126] In the embodiment of FIG. 35, a lateral alignment spring 519 issupported by a vertical beam portion 520 extending upwardly from thefloor of trench 432 in place of the serpentine springs of FIG. 34. Inthis embodiment, the spring 519 includes an alignment tip 522 and aretractor ring 524 fabricated in the trench 432 by the SCREAM-1 processdescribed above. The alignment tip 522 is released for motion withrespect to substrate block 434 by a back surface deep etch indicated bydotted line 526, while the retractor ring portion 524 is released by aback surface deep etch in the location indicated by dotted line 528. Theback surface etching at 526 and 528 are separated by a vertical beamportion 520 which is flexible to allow the edge alignment tip 522 tomove in the trench in directions from right to left as viewed in FIG.35. Retractor pin 500 may be used to engage the ring 524 to press thealignment spring toward the right to permit the substrate 430 to bemounted in trench 432 or to permit its release. The lateral springs ofFIGS. 32-33 engage the end of substrate 430 in the manner illustrated byarrow 438 in FIG. 27, or engage the front edges of the substrate in themanner illustrated by arrows 490 in FIG. 31.

[0127] As diagrammatically illustrated in the top plan view of FIG. 36,edge mounted substrate 430 can be secured in trench 432 of coupler block434 by angled corner alignment springs such as those illustrated byarrows 530 and 532. When corner alignment springs are used, thesubstrate is urged away from the etched fiber guides on substrate 434and is held with the back wall 534 of the substrate engaging the wall oftrench 432. This eliminates the need for the back wall alignment spring436 of FIG. 30 and changes the direction of force supplied by the edgealignment springs 438 and 490 of FIG. 31. By placing the back surface534 of substrate 430 in contact with the coupler block 434, heattransfer from optoelectronic components on the substrate 430 isimproved.

[0128] Exemplary corner alignment spring structures are illustrated intop plan view in FIGS. 37, 38 and 39. These are similar to the lateralalignment springs previously discussed, but as noted above, provide analignment force at an angle against the front and side edges of thesubstrate. Thus, as illustrated in FIG. 37, trench 432 in coupler block434 is shaped to receive a corner alignment spring 530 which includes ofa corner alignment tip 536 supported by a pair of spring arms 538 and540 connected in cantilever fashion to the wall of trench 432 in block434 at 542. The corner alignment spring 530 also includes a retractorring 544 which may be located within spring arms 538 and 540 and whichis adapted to receive the retractor pin 500 for shifting the alignmenttip 536 away from the corner of substrate 430. As illustrated by dottedline 546, the alignment spring 530 is released by a rear surface etch inthe manner previously discussed. The corner alignment tip 536 is notchedas at 548 to engage a forward corner of substrate 430 to ensure propercontact between the alignment tip 536 and the substrate 430.

[0129] Another form of the corner alignment spring 530 is illustrated inFIG. 38, at 548 wherein the serpentine spring of FIG. 34 is angled sothat its alignment tip 510 engages the front corner of substrate 430. Asillustrated in FIG. 38, tip 510 may incorporate a notch 550, asdiscussed with respect to FIG. 37.

[0130] A variation in the structure of FIG. 38 is illustrated in FIG. 39wherein the comer alignment tip 510 of spring 548 is outwardly tapered,as at 552, to engage a corresponding notch 554 in the front corner ofsubstrate 430. The notch 554 preferably is etched in substrate 430 andcan be accurately positioned on the edge-mounted substrate duringlithographic patterning so as to provide improved vertical andhorizontal alignment of substrate 430 with the optical fibers carried bythe coupler block 434. Similarly, the alignment spring 548 islithographically aligned with the fiber guide structures so that whenthe alignment spring 548 engages the notch in the substrate 430, preciselateral alignment of the edge-mounted substrate to optical fibers heldin the fiber guides is attained.

[0131] Another embodiment of the present invention is illustrated indiagrammatic form in the top plan view of FIG. 40, wherein the opticalcoupler block 434 incorporates alignment trench 432 for receiving theedge-mounted substrate 430 to be secured in alignment with fiber guides442, as previously discussed. In this embodiment, the substrate 430 isheld in alignment by a pair of front surface alignment springs generallyindicated at 560 and 562 in place of the previously-described lateral,edge and rear alignment springs. In this embodiment, the springs 560 and562 engage a front surface 564 of the substrate to press it rearwardlyaway from the optical guides 442 so that its rear wall 534 is inengagement with a rear wall 566 of the precision etched trench 432.These springs position the substrate and provide heat transfer betweenthe substrate and the coupler block of 434, while allowing expansion andcontraction of the substrate 430 due to temperature changes. Lateralalignment of substrate 430 within trench 432 horizontally (toward theleft and toward the right as viewed in FIG. 40) and vertically isobtained by precision etching of the front alignment springs 560 and 562and by providing alignment tips on the springs which engagecorresponding alignment notches (or pits) 568 and 570 which are alsoprecision etched on the face 564 of the substrate.

[0132] The front alignment springs 560 and 564 may take a variety offorms, as illustrated in FIGS. 41-47, but all incorporate a tapered tipor notch, or both, for ensuring accurate lateral alignment. Thus, forexample, in FIG. 41 the front alignment spring 562 incorporates atapered alignment tip 572 supported in trench 432 by a pair of springarms 574 and 576. These arms are mounted as cantilevered beams on a wallof trench 432 and are released for motion with respect to coupler block434 by a deep trench etch on the back side of the block 434, asillustrated by the dotted line 578. The alignment spring 562 may bemoved forwardly and rearwardly by the retractor pin 500 to permitinsertion of substrate 430 into trench 432, and to release the substratewhen desired. The forward end of tip 572 is tapered, as illustrated at580, so that when it engages notch 570 under the urging of spring arms574 and 576, the tip laterally aligns the substrate 430 while securingit in position. It will be understood that the alignment spring 560 ispreferably a mirror image of spring 562.

[0133] The alignment spring 562 is shown in its rest position with thesubstrate 430 removed from trench 432, although the location of thesubstrate is indicated in dotted lines. FIGS. 42-47, on the other hand,show the substrate 430 in place and illustrate a variety of frontalignment springs engaging corresponding notches on the front surface564 of the substrate. Thus, for example, in FIG. 42 a modified alignmentspring 581 includes a tip 582 which is similar to that of FIG. 41, butis wider and shorter. As illustrated, the tip 582 has a double taper 584on its forward end to engage both sides of notch 570. The tip is urgedtoward the substrate by its spring arms 583 for centering the notch andlaterally aligning the substrate 430, since the spring arms are flexibletoward and away from the substrate, but are relatively inflexible in adirection parallel to the face 564 of the substrate. In FIG. 43, spring585 includes an alignment tip 586 which has a single taper on itsforward end 588 to engage one side of notch 570. This taper tends topress the substrate toward the left, as viewed in this figure, while itscorresponding alignment spring on the other end of the substrate willtend to press the substrate toward the right, thereby aligning thesubstrate.

[0134]FIG. 44 illustrates a modified alignment spring 589 carrying analignment tip 590 having a tapered forward end 592 which tends to presssubstrate 430 toward the right. This spring operates in cooperation withits corresponding spring on the opposite end of the substrate tolaterally align it.

[0135] In the embodiment of FIG. 45, spring 593 incorporates analignment tip 594 having a V-shaped end 596 which engages a locator beam598 extending outwardly from the base of notch 570, the V-shaped endtending to align substrate 430 by engaging beam 598.

[0136] In FIG. 46, the alignment spring 599 incorporates a tip 600 whichhas a flat forward end 602. In this case, the notch 570 is V-shaped sothat it provides outwardly tapered walls which are engaged by spring tip600 to laterally align substrate 430. Finally, in FIG. 47, the alignmentspring 603 incorporates an alignment tip having a double taper on itsforward end 606 which engages a V-groove notch 570 to provide lateralalignment of substrate 430.

[0137] In the foregoing embodiments, the alignment tips for the forwardsurface alignment springs are keyed to corresponding notches in theedge-mounted substrate 430 so that variations in etched sidewallpositions for both the alignment tip and the notch are automaticallycompensated for, since the tapered alignment tip always centers on theetched notch. As noted above, the alignment springs are designed to berelatively stiff in the lateral direction, although some flexibility isrequired to allow for the effects of thermal expansion.

[0138] The alignment structures described above in FIGS. 28-39, forexample, provide lateral alignment of the edge-mounted substrate in aplane parallel to the surface of the coupler block 434. This alignmentis from left to right or from front to back in the top plan views ofthese figures, as described above. Alignment of the substrate in couplerblock 434 in a direction perpendicular to the surface of block 434 isdetermined in the foregoing embodiments by the depth of the deep etchtrench and by precision cleaving of the edge-mounted substrate fordetermining the precise dimensions of the substrate. However, by etchingnotches in the edge-mounted substrate, alignment springs can provideboth lateral alignment in the plane of the top surface of the opticalcoupler block and alignment in a direction perpendicular to thatsurface. FIG. 48 illustrates a structure for lateral and perpendicularalignment of an edge-mounted substrate such as substrate 430 in acoupler block 434, and is a perspective cutaway view of the coupler,illustrating trench 432 formed in block 434 to receive substrate 430 andto receive alignment springs such as those generally illustrated at 620and 622. As illustrated, alignment spring 620 includes an alignment arm624 mounted in trench 432 by flexible spring arms 626 and 628, the armsbeing positioned to urge alignment arm 624 in a direction to engage theedge-mounted substrate 430 to hold the substrate in place within thetrench 432. In this embodiment, the substrate 430 incorporates an etchednotch 630 which receives a tip portion 632 of arm 624 to align thesubstrate 430 both horizontally, in the plane of the top surface 634 ofblock 434, and vertically, in a direction perpendicular to the plane ofsurface 634.

[0139] The alignment spring 620 is fabricated in the silicon block 434utilizing the SCREAM-1 process, for example, and is released for motionwith respect to block 434 by a back side deep etch, generally indicatedby cavity 640 formed through the back surface 642 of block 434. It willbe understood that the alignment spring 620 can be retracted by asuitable spring retractor pin 642 to withdraw tip 632 from notch 630 torelease the substrate 430.

[0140] Spring 622 may be a mirror image of spring 620, if desired, butfor purposes of illustration it is shown as incorporating a support 644carrying an alignment arm 646 having a tip 648 which engages a notch 650in the end of substrate 430. The support 644 is carried by a pair ofspring arms 652 and 654 which are mounted in cantilever fashion to theside wall of trench 432. These spring arms 652 and 654 are released formotion with respect to coupler 434 by a back side deep etch, indicatedat 640, as previously described. The spring arms 650 and 652 urge thetip 648 into engagement with notch 650 and may be retracted by aretractor pin engaging the support 644. The notches 630 and 650 arecarefully located on the substrate 430 to position the substratevertically for alignment with optical fibers, as previously described,and to align the substrate horizontally side to side and front to backin the plane of the top surface 634 of coupler 434 so that the substrateis precisely and positively positioned in the coupler.

[0141] FIGS. 49-57 show several different edge-mounted substrate notchdesigns which may be used with the alignment springs of the deviceillustrated in FIG. 48. In FIGS. 49-52, edge notches 660, 662, 664 and666, respectively, are illustrated. Notch 660 is shown without a taper,and thus is similar to notches 630 and 650 in FIG. 48. Notches 662, 664and 666 include one or more tapered surfaces which serve to guide thespring-loaded alignment tips of the alignment springs so that theedge-mounted substrate 430 will be aligned with the middle, top, orbottom of the alignment tip. For example, notch 664 provides accuratealignment with the top surface of an alignment tip and may beincorporated in an edge-mounted substrate which is designed to bealigned with a surface substrate waveguide.

[0142] As illustrated in FIGS. 53-57, the edge notches can be extendedto provide alignment ramps which engage the corresponding springalignment tips as the substrate 430 is inserted into the trench 432.Thus, an alignment ramp 670 is provided for each of the notches 660,662, 664 and 666 to guide the alignment tips into the notches as thesubstrate 430 is pressed downwardly into the trench 432. The location ofthe alignment ramp 670 is illustrated in FIG. 48 by a dotted line on theend surface 656 of the substrate. This ramp arrangement reduces thenumber of retractor pins required, thereby simplifying assembly of thesubstrate and the coupler. In the embodiments of FIGS. 54-57, anintegrated shoulder is provided to serve as a catch for the alignmentspring tip to hold the substrate 430 captive after assembly. Middle, topand bottom surface alignment tapers for the notches can be used, as alsoillustrated in FIGS. 50-52.

[0143] If desired, the notches 630 and 650 can be moved away from theedge of substrate 430, either onto the end wall 656 or onto the frontsurface 564 (FIGS. 40 and 48). If the notches are placed on the end wall656, they will be engaged by end alignment springs 620 and 622, whereasif they are placed on the front surface 564, they may be engaged byalignment springs such as spring 562 illustrated in FIG. 41. End orfront surface notches on substrate 430 are illustrated in FIGS. 58-67 asbeing located on end surface 656. In each of the illustratedembodiments, a surface notch 680 is connected to a tapered alignmenttrench 682, as shown, for example, in FIG. 58. The alignment trench istapered inwardly and upwardly from the bottom edge of the substrate 430to the notch 680. In each of the embodiments illustrated in FIGS. 58-67,the alignment trench guides the alignment spring tip into thecorresponding notch, with most of the illustrated embodimentsincorporating a shoulder, or catch 684 (See FIG. 59) which secures thetip in place. One advantage of utilizing the trenches of FIGS. 58-67instead of the notches of FIGS. 49-57 is that the trenches are lesslikely to suffer damage during cleaving of the substrate 430.

[0144] The notch and trench configuration of FIG. 62 is illustrated inperspective view in FIG. 68. In this embodiment, the notch is located onthe front surface 564 of substrate 430 and receives a tapered alignmenttip 686 mounted on an alignment spring structure such as thatillustrated in FIG. 41. In this embodiment, when the substrate ispressed into trench 432, the alignment tip 686 enters the alignmenttrench 682 and slides upwardly into the notch 680 where it latches overthe shoulder 684 to prevent removal of the substrate. This arrangementpositions the substrate both vertically and horizontally within trench432, as discussed above.

[0145] The tapered alignment tip 686 (FIG. 68) automatically compensatesfor variations in etched notch locations, and FIGS. 69-72 illustrateseveral etched trench and notch designs for use with tapered alignmenttips. Thus, for example, FIG. 69 illustrates a tapered notch 690 whichis V-shaped in cross section to engage the tapered tip 686. An alignmenttrench 692 leads to the V-shaped notch, 690 to guide the alignment tip686 into place as the substrate 430 is positioned in trench 432. Insimilar manner, FIGS. 70, 71 and 72 illustrate variations in theV-shaped notch 690 for receiving and securing the alignment tip 686.These variations are illustrated at 694, 696 and 698, respectively, andillustrate that the V-groove notch can be combined with deep or shallowetches to create alignment channels which provide middle, bottom and topsubstrate alignment.

[0146]FIG. 73 is a perspective view of the bottom alignment v-groovearrangement of FIG. 71, wherein a deep etch tip stop 700 provides bottomalignment for an alignment tip 702 mounted on a spring arm 704. Thealignment tip 702 has a square end that is aligned horizontally by theV-groove 696 and vertically by the tip stop 700. A shallow etch tipguide 706 guides alignment tip 702 into the groove 696 as the substrate430 is inserted into trench 432.

[0147] As illustrated in the top view of FIG. 74, wherein a top portionof the substrate 430 is cut away to illustrate alignment notch 710 onthe face of the substrate, the notch 710 is etched isotropically so thatthe width of the trench at the surface of substrate 430 is narrower thanthe width of the trench at its bottom. With this arrangement, anoutwardly flared alignment tip 712, mounted on spring arm 714, can beinserted into the trench from a downwardly extending tip guide trench sothat the notch will hold the alignment tip captive upon completeinsertion. FIG. 75 shows a smaller version of the alignment notch ofFIG. 74, with a notch 716 being smaller in width than the notch 710 andan alignment tip 718 being smaller than the alignment tip 712. FIGS. 76and 77 illustrate two forms of the notch 710, and show how the frontsurface width 720 is narrower than the rear width 722 (shown in dottedlines). In the configuration of FIG. 77, the notch includes a shoulderportion 724 for securing the alignment tip. FIG. 78 is a partialperspective view of the alignment trench 710 of FIGS. 74 and 76.

[0148] As illustrated in FIGS. 79-84, the alignment tips described abovecan be provided with one or more burrs fabricated at the ends of thetips to provide permanent attachment of the edge-mounted substrate 430to the optical coupler. As illustrated in FIG. 79, for example,alignment tip 730 may be provided with burrs 732 and 734 attached at theforward end 736 of the tip 730 and extending outwardly and rearwardly.When the tip is inserted into a notch, or etched trench, 738 formed inthe face of an edge-mounted substrate 430, the rearwardly and outwardlyextending burrs will engage the side walls of the trench to preventremoval of the tip. The notch, or trench, 738 can be etchedisotropically to have flared walls 742 as illustrated in FIG. 79, mayhave perpendicular walls as illustrated at 744 in FIG. 80, or may haveoutwardly tapered walls 746 as illustrated in FIG. 81. FIGS. 82, 83 and84 illustrate rearwardly and outwardly extending burrs 748 on both sidesof tip 750, or on the right hand or left hand edges of the tip 750,respectively.

[0149]FIG. 85 is a perspective view of the device of FIG. 79,illustrating how a burred alignment tip would meet with a slightlyisotropically etched alignment trench on the surface of an edge-mountedsubstrate. This figure also illustrates an optional through hole 760extending from the forward surface 762 of substrate 430 to the rearsurface 764. If a through-hole such as the hole 760 is etched in thesubstrate 430, alignment tips can be used to support the substrate fromboth sides, as illustrated in FIG. 86. This approach would be useful ifthe edge-mounted substrate is to be mounted so that it is free fromcontact with the sidewalls of trench 432, as might be the case if thesubstrate 430 were to be attached to a micropositioner or a micromotorfor optical alignment or switching. Thus, for example, in FIG. 86 thesubstrate 430 is provided with a through-hole 766 which is engaged atthe front surface 762 of substrate 430 by a first alignment tip 768 andwhich is engaged at the rear surface 764 by a rear alignment tip 770. Inthis embodiment, both alignment tips are mounted by corresponding springarms 772 and 774, respectively, to the edge wall of trench 432, aspreviously discussed. As illustrated in FIG. 87, flared alignmenttrenches 776 on the front and back surfaces of substrate 430 may lead tothe etched through-hole 766 to facilitate assembly of the substrate 430with the alignment tips 768 and 770. FIGS. 88-91 illustrate a variety ofshapes for through-holes 780, 782, 784 and 786, respectively, each ofwhich is connected to an alignment trench for guiding the alignment tipsinto place. FIG. 92 is a perspective view of the device of FIG. 86, withcommon features carrying common identifying numerals.

[0150] Another modification of the coupler of FIG. 28 is illustrated inFIG. 93, to which reference is now made. As previously discussed, thecoupler block 434 includes a trench 432 for receiving an edge mountedsubstrate 430. The substrate 430 carries a plurality of active elementssuch as VCSEL sources 780 mounted on a front surface 782 of substrate430, each of the VCSEL sources being connected to a corresponding wirebond pad 784 mounted on front surface 782 in the manner describedhereinabove. The coupler block 434 incorporates a plurality of etchedfiber guides 442 which receive optical fibers 408, as previouslydescribed. To align the VCSEL sources with the corresponding fiberoptics in this embodiment, the substrate 430 incorporates a plurality ofvertical alignment trenches 786 and 788 formed on the lower edge 790 ofthe substrate. These vertical alignment trenches engage correspondingself alignment stops such as the stop 792 formed in the trench 432 toalign the VCSEL sources in a direction perpendicular to the top surface794 of coupler block 434. Horizontal alignment of substrate 430 isprovided by an edge alignment trench 796 such as that described withrespect to FIG. 58 for receiving an edge alignment spring 798 carried bycoupler block 434. As illustrated, the coupler block also carries a backsurface alignment spring 800 for engaging a corresponding alignmenttrench in the back surface 802 of substrate 430 in the manner describedabove. In this embodiment, the optical fibers 408 are secured in theetched fiber guides 442 by means of fiber alignment finger springs 804,such as those described at 352 and 354 in FIG. 18. In addition, couplerblock 434 preferably incorporates a plurality of wire bond pads such asthe pads 428 of FIG. 26.

[0151] The fiber alignment fingers 804 grip the optical fiber to secureit in place. Although these fingers are illustrated as being relativelythin and flexible, it will be understood that deeper alignment fingersmay be provided, such as the fingers 806 and 808 illustrated in FIG. 94.Since the active elements 780 can be placed below the top surface 794 ofcoupler 434, in accordance with the present invention, the fiber guides442 can be sufficiently deep that the fingers 804 will secure the fibers408 in place, as illustrated in FIGS. 94 and 95. As illustrated indotted lines, the bottom edge notch 788 rests on a verticalself-alignment stop 792 fabricated during the same etch that is used tocreate the fiber guide 442.

[0152] As previously described, the alignment fingers 804 are fabricatedutilizing the SCREAM-1 process for undercutting and releasingcantilevered beams. An alternate structure for the fiber alignmentfinger springs and the vertical alignment stop is illustrated in FIGS.96, 97 and 98. In this arrangement, the coupler block 434 is etchedthrough the top surface 794 to form a multiplicity of fiber alignmentfinger springs 810. These fingers are then released by a back surfaceetch 812 which leaves a bottom fiber guide wall 814. The fingers havehigh aspect ratios so as to be flexible in the horizontal direction andrelatively inflexible in the vertical direction, to enable them toreceive and secure optical fibers 408. It will be noted that in thisprocess the vertical alignment stop 792 is fabricated during theisotropic etch used to form the fingers 812 and the underlying fiberguide 814.

[0153] In addition to securing edge-mounted substrates in a couplerblock, it will be apparent that the alignment springs of the presentinvention can be utilized to secure a variety of components on asubstrate. Thus, for example, as illustrated in FIG. 99, alignmentsprings 820 and 822 may be fabricated in the surface of a coupler block824 in the manner described above with respect to FIG. 38 to secure anedge-mounted substrate 826 in alignment with an etched fiber guide 828.In this case, the fiber guide may also be aligned with a ball lens 830secured by 4 additional alignment springs 832, 834, 836 and 838 locatedin a corresponding deep etch trench 840. Other components may similarlybe aligned with each other and with etched fiber guides, utilizing anyof the alignment spring configurations described herein. Thus, forexample, FIG. 100 illustrates another embodiment wherein a substrate 840is mounted parallel to the top surface 842 of a coupler block 844 by aplurality of alignment finger springs 846 and 848. The substrate 840 maybe held in alignment with an etched fiber guide 850 which is alsoaligned with a GRIN lens 852, also held in place by alignment fingersprings 854 and 856. In this embodiment the optical fiber carried inetched fiber guide 850 may be secured in place by a suitable epoxy. Forthis purpose, a trench 862 is provided in the top surface 842intersecting the path of the fiber guide 850 so that when the opticalfiber is in place, epoxy can be poured into the trench to hold the fiberin place.

[0154] FIGS. 101-107 illustrate various electrical interconnectionsbetween edge-mounted substrates such as the substrate 430 and opticalcoupler blocks such as the block 434 described above. Optoelectronicdevices require reliable electrical connections, and the edge-mountedsubstrates present unique challenges because they deviate from theplanar mounting approach which dominates electronic packaging. Thetechniques described in FIGS. 101-107 are applicable to the variousconfigurations described hereinabove, as well as to other electronicpackaging techniques.

[0155] Wire bonding is commonly used to provide connections betweenchips and their packages. This approach is modified for edge-mountedsubstrates by a bonding technique that connects surfaces which are atright angles to each other, as illustrated in FIG. 101. In this case,the edge-mounted substrate 430 and the optical coupler block 434 carrycorresponding connector pads 870 and 872 which are interconnected by anL-shaped wire bond connector 874. In conventional manner, connector 874is secured to pads 870 and 872 by solder droplets 876 and 878,respectively.

[0156] Flip-chip solder ball interconnections are becoming more commonin electronic packaging, and a variation on the standard method can beused for edge-mounted substrates. Thus, as illustrated in FIG. 102, inplace of the wire bond connection of FIG. 101, solder balls 880 and 882are initially deposited on the connector pads 870 and 872, respectivelylocated on the edge-mounted substrate 430 and the connector block 434.The solder balls on the substrate can then be reflowed with thesubstrate 430 assembled in the connector block 434, as described above,so that the two solder balls melt together to form an electricalconnection as generally indicated at 884 in FIG. 103. The geometries ofthe solder ball volume and of the connector pad are adjusted to minimizethe effects of thermal expansion.

[0157] If desired, stresses due to thermal expansion can be relieved bymounting the solder ball interconnection for the coupler block 434 on areleased cantilevered beam spring 890. The spring may move with theexpanding or contracting edge-mounted substrate during temperaturechanges to thereby relieve stress on the solder connection. The beam canalso be formed with a serpentine stress-relieving spring along itslength to compensate for changes in length due to expansion orcontraction of the edge-mounted substrate.

[0158] As illustrated in FIG. 105, the stress-relieving beam 890 mayalso be used in the flip-chip mounted substrates illustrated in FIG. 4,for example, with the beam 890 supporting a flip-chip mounted substratesuch as the substrate 76. In this case, the solder ball 82 of the deviceof FIG. 4 is secured to pad 84 mounted on beam 890.

[0159] Electrical contacts between edge-mounted substrates and thereceiving optical coupler block 434 can be created by metallizing aflexible released beam 900, as illustrated in FIG. 106, the metal layerforming a connector lead 902 which covers a tip of the beam to form ametal tip 904. This metal tip may contact a metallized pad 906 on thesurface of substrate 430 to provide the desired electrical connection.The beam then compensates for differences in thermal expansion betweenthe substrate 430 and the coupler block 434. The SCREAM-1 processprovides metalization on the tops and side walls of released beamswithout electrical contact to the underlying silicon of the couplerblock 434, thereby allowing the side walls of the tip 904 to bemetallized to provide a reliable connection. Since thermal expansioneffects would tend to drag the tip contact over the surface of pad 906,an etched alignment trench 908 can be provided in the surface ofsubstrate 430. An alignment tip 910 may then be fabricated on thecontact beam 900 to engage the trench 908 to stabilize the contact pointand to prevent the beam from shifting with respect to the substrate 430.

[0160] Thus there has been described a unique electronic packagingsystem for interconnecting fiber optics with waveguides, active opticalelements, and other optical components mounted on a coupler block.Although the application of the principles of the present invention havebeen illustrated in numerous embodiments, various other modificationswill be apparent to those of skill in the art. Accordingly, the presentinvention is limited only by the following claims.

What is claimed is:
 1. An optical coupler, comprising: a coupler block;at least one fiber guide in said block for receiving an optical fiber;and means on said block for transferring light between an optical fiberin said guide and an optical element.
 2. The optical coupler of claim 1,wherein said optical element is a second fiber guide for receiving asecond optical fiber axially aligned with said first fiber guide.
 3. Theoptical coupler of claim 1, wherein said optical element is located on asubstrate edge-mounted in said optical block.
 4. The optical coupler ofclaim 1, wherein said optical element is a waveguide.
 5. The opticalcoupler of claim 1, wherein said optical element is an active element.6. The optical coupler of claim 1, wherein said optical element is asubstrate surface mounted on said block.
 7. The optical coupler of claim1, wherein said means comprises an elongated trench in said blockbetween said fiber guide and said optical element.
 8. The opticalcoupler of claim 1, wherein said means comprises a reflector integralwith said block.
 9. The optical coupler of claim 1, wherein said meanscomprises a waveguide having a first end aligned with said fiber guideand a second end aligned with said optical element.
 10. The opticalcoupler of claim 9, wherein said waveguide is embedded in said block.11. The optical coupler of claim 9, wherein said waveguide is located onthe surface of said block.
 12. The optical coupler of claim 1, whereinsaid optical element is a second fiber guide coaxial with said at leastone fiber guide for receiving a corresponding optical fiber, and whereinsaid means for transferring light includes an intermediate guide portionbetween said fiber guides, and further including a fiber stop in saidintermediate guide portion.
 13. The optical coupler of claim 12, furtherincluding a flared end portion for each fiber guide for facilitating theinsertion of optical fibers.
 14. The optical coupler of claim 13,wherein said block is single crystal silicon and wherein said fiberguides and said intermediate guide portions are etched in said block.15. The optical coupler of claim 14, wherein said at least one fiberguide and said second fiber guide comprise a fiber guide pair, andwherein said coupler block includes a multiplicity of fiber guide pairsfor coupling optical fiber arrays.
 16. The optical coupler of claim 1,wherein said optical element is an active element mounted on a substrateedge-mounted in said optical block with said active element aligned withsaid fiber guide.
 17. The optical coupler of claim 16, wherein saidactive element is a light emitter.
 18. The optical coupler of claim 16,wherein said active element is a light detector.
 19. The optical couplerof claim 16, further including circuit means on said block andelectrically connected to said active element.
 20. The optical couplerof claim 1, wherein said coupler block has a top surface, said at leastone fiber guide having a longitudinal axis located in said block belowsaid surface; further including an elongated, narrow, deep trench insaid block and perpendicular to said fiber guide axis; a substrateedge-mounted in said trench and having a wall perpendicular to saidfiber guide axis, said optical element comprising an active opticalelement mounted on said substrate wall; and alignment means in saidtrench for aligning said optical element and said longitudinal axis ofsaid fiber guide.
 21. The optical coupler of claim 20, further includingmultiple optical guides in said coupler and multiple active opticalelements on said substrate wall, said optical elements corresponding tosaid fiber guides, wherein said alignment means in said trench alignssaid optical elements and the longitudinal axes of corresponding fiberguides.
 22. The optical coupler of claim 20, wherein said alignmentmeans comprises a precision-etched trench wall perpendicular to saidfiber guide axis.
 23. The optical coupler of claim 20, wherein saidalignment means includes at least one microspring in said trench forengaging said substrate.
 24. The optical coupler of claim 23, whereinsaid microspring is fabricated from and is integral with said block. 25.The optical coupler of claim 24, wherein said microspring is amicromechanical cantilever beam structure extending into said trench toengage said substrate.
 26. The optical coupler of claim 25, wherein saidmicrospring includes a retractor for releasing said substrate.
 27. Theoptical coupler of claim 23, wherein said microspring is located in saidtrench to engage an edge of said substrate.
 28. The optical coupler ofclaim 23, wherein said microspring is located in said trench to engage aback surface of said substrate.
 29. The optical coupler of claim 23,wherein said microspring is located in said trench to engage a corner ofsaid substrate.
 30. The optical coupler of claim 23, wherein saidmicrospring is located in said trench to engage a front surface of saidsubstrate.
 31. The optical coupler of claim 23, wherein said substrateincludes at least one positioning notch for receiving said microspring.32. The optical coupler of claim 31 wherein said notch is tapered toaccurately position said substrate with respect to said microspring. 33.The optical coupler of claim 31, wherein said notch includes a V-groovefor positioning said substrate.
 34. The optical coupler of claim 31,wherein said notch is precision etched on a front surface of saidsubstrate, and wherein said microspring includes a tip for engaging saidnotch.
 35. The optical coupler of claim 20, wherein said alignment meansincludes at least one stop precision etched in said trench for engagingsaid substrate.
 36. The optical coupler of claim 20, further includingelectrical contact pads on said substrate and on said coupler block forelectrically connecting said active optical element on said substrate tocircuitry on said block.
 37. The optical coupler of claim 36, furtherincluding a wirebond connection between said contact pads.
 38. Theoptical coupler of claim 36, further including reflowed solder ballsinterconnecting said contact pads.
 39. The optical coupler of claim 38,wherein said contact pad on said coupler block is mounted on a releasedcantilevered microstructure beam.
 40. The optical coupler of claim 36,wherein said contact pad on said coupler block is mounted on an end tipof a released, cantilevered microstructure beam, said contact pad onsaid tip engaging said contact pad on said substrate.
 41. The opticalcoupler of claim 1, wherein said fiber guide is an etched trench in saidcoupler block, said coupler further including multiple fiber alignmentfingers along said trench to receive and secure an optical fiber in saidguide.
 42. The optical coupler of claim 1, wherein said optical elementis an active element located on a surface-mounted substrate, and whereinsaid means for transferring light comprises a reflector integral withsaid block.
 43. The optical coupler of claim 42, wherein said reflectoris a precision etched reflective surface in said block, said surfacebeing aligned with said fiber guide and with said active element. 44.The optical coupler of claim 1, wherein said optical element is anactive element mounted on the edge of a substrate, said substrate beingsurface-mounted in a cavity on said block, said cavity beingsufficiently deep to align said active element with said fiber guide.45. The optical coupler of claim 1, wherein said optical element is afirst waveguide mounted on said block, and wherein said means fortransferring light is a deep-etched second waveguide fabricated in saidblock and having a first end aligned with said fiber guide and a secondend aligned with said first waveguide.
 46. The optical coupler of claim45, further including a lens between said fiber guide and said secondwaveguide.
 47. A micromechanical optical coupler for interconnecting asmall-diameter optical fiber with an optical element, comprising: asingle crystal silicon block; a fiber guide trench etched through a topsurface of said block, said guide trench adapted to receive an opticalfiber; and a receptacle etched through the top surface of said block forreceiving a substrate carrying active optical elements to be opticallycoupled to an optical fiber in said fiber guide, said receptaclecomprising a precision etched trench intersecting said guide trench 48.The optical coupler of claim 47, said receptacle containing at least onereleased beam microspring integral with said block and cantilevered on awall of said trench, said microspring having a movable free end adaptedto engage and secure a substrate in said receptacle.
 49. The opticalcoupler of claim 48, said block carrying electrical circuit means, andcontacts on said block on said substrate which are connectable toelectrically connect said optical element to said circuit.
 50. Theoptical coupler of claim 49, said microspring including release meansfor releasing said substrate.
 51. A method of fabricating amicromechanical optical coupler in a single crystal silicon blockcomprising: patterning a top surface of the silicon block;anisotropically etching a first trench in said top surface to define thelocation of a fiber guide channel extending the length of said block;isotropically etching through said first trench a continuous subsurfacefiber guide channel extending the length of said first trench, saidguide channel having first and second ends terminating at first andsecond end walls of said block; and controlling the diameter of saidguide channel to receive and hold first and second optical fibersinserted into said first and second ends of said guide channel,respectively, said optical fibers being axially aligned with each otherby said guide channel.
 52. A method for fabricating a micromechanicaloptical coupler in a single crystal silicon block, comprising:patterning a surface of said block to define the location of a fiberguide channel and an intersecting trench; and etching through saidpattern to produce a subsurface fiber guide channel and an intersectingtrench open to the surface of said block, said fiber guide channel beingadapted to receive an optical fiber and said intersecting trenchpermitting coupling of light between a fiber in said guide channel andan optical element.
 53. The method of claim 52, further includingmounting on said block a substrate carrying said optical element. 54.The method of claim 53, further including fabricating a reflectivesurface in said intersecting trench.
 55. The method of claim 53, furtherincluding fabricating a waveguide in said intersecting trench.
 56. Themethod of claim 55 further including fabricating microsprings in saidintersecting trench; inserting a substrate carrying an optical elementin said trench; and engaging said substrate with said microspring tosecure said substrate and align said optical element with said guidechannel.